Sorry - some of that might sound too techy
As already mentioned there is computer lag and internet lag.
As far as I know MJ (and I have dealt with it quite a bit) it does NOT cause internet lag on cable-, and DSL lines (i.e. non-modem lines).
Although there is a steady data transfer between your plugin and my server, the amounts are by far smaller and the transfers by far less frequent than what your AC client and the AC server exchange.
56K lines are a special problem, because in fact every single byte that leaves the modem means lag
Computer lag is a different topic. The speed of your CPU and the amount of RAM you have determine whether any active plugin takes resources that your AC client might need to run smoothly. Even the speed of your harddrive and the size/location of your Pagefile might have an influence.
For instance the fastest cable connection cannot prevent lag if your PC only has 64MB RAM or a you have Pentium3/500 - or both - and you run a firewall, an anti-virus software, ACClient, Decal, ElTank, MJ, ACStats and maybe even more (video card speed is not too important in this respect).
Another aspect is the dependency of plugins, filters and their versions.
MJ is an ImpFilter-user. As are ACStats, Housekeeper, ImpInventory and others. If you have an "unsupported combination" of ImpFilter and those plugins this may cause lag within ImpFilter itself or one of the plugins.
For instance if MJ requires ImpFilter version X but ImpInventory needs and installed ImpFilter version Y, you cannot blame MJ if it acts weird.
If you enable MJ and experience lag that you definitely don't see when turning MJ off, MJ obviously causes the lag but it is not necessarily the reason
As we can most likely eliminate connection lag we would have to analyze your PC a bit more.....